1. Field of the Invention
The invention relates to a radio receiver for a phase or frequency modulated information signal, comprising an analog receiving section which provides non-linear dynamic compression, an analog-to-digital converter and a digital signal processing section.
Radio receivers of this type can be used, for example, in the Pan-European mobile radio system GSM. In this mobile radio system speech signals in digitized form are transmitted together with other digital signals in a Time-Division Multiple Access mode (TDMA).
2. Description of the Related Art
DE 39 25 305 A1 has disclosed a radio receiver of the type mentioned in the opening paragraph. In that document the frequency of a received information signal is selected in an analog receiving section of the radio receiver and converted into an intermediate frequency by means of a mixer. An IF amplifier which performs non-linear dynamic compression is connected to the IF mixer. As a result, the output signal of the IF amplifier is either compressed or limited, depending on the input signal level. The output signal of the IF amplifier is applied to a baseband converter which produces therefrom two quadrature components. These quadrature components are sampled and applied to an analog-to-digital converter. Subsequently, the sampled digital values are buffered in a digital signal processing section, a channel estimate is made and an equalization and a decoding is performed. The non-linear dynamic compression and limitation achieves that a more cost-effective analog-to-digital converter can be used, having a relatively small number of quantization steps. Even at the largest possible levels of the input information signal there is no overload of the analog-to-digital converter, and, nevertheless, even at the smallest possible input levels usable sample values are still produced. However, in practice is found that the output signal of the digital signal processing section has bit errors.